Korean deposits have a huge range depending on the location and type of property. Generally, the higher the deposit, the less monthly rent you have to pay.
When I was looking for housing in Seoul, the lowest housing deposit I could find for a decent one-room was 5,000,000. However, there really weren’t too many options and housing is getting more expensive. I would aim for a 10,000,000 won deposit.
As for paying you back at the end of the contract. I have never heard of anyone not getting their deposit back. If they happened you could contact the Seoul Global Center, or your local foreigner office and they would assist you with finding a lawyer.